In the autofocusing systems of the prior art, photodiodes are arranged as position sensitive detectors (hereinafter PSDs). A spot of light is directed at the subject being photographed, videographed or located, and the PSD receives a reflected light spot. The PSD generates current that is used to develop a linear position value. Typically, the PSD is a rectangularly shaped PN junction device with two output electrodes, one at each end of the rectangle.
FIG. 1 depicts an example of a prior art PSD photosensor 1. Contact 9 couples output terminal I1 to the left end of a p type semiconductor layer 3. Contact 11 couples output terminal I2 to the right end of p type semiconductor layer 3. The p type layer 3 is spaced apart from the n type semiconductor layer 5 that makes up the N part of the PN junction. A high resistivity Silicon substrate material 7 lies between the p type layer 3 and the n type layer 5. The n type layer 7 is coupled to a ground or common terminal.
In operation, when a spot of incident light strikes the photosensor of FIG. 1, the device outputs two currents at the output terminals I1 and I2. The incoming photons produce electron-hole pairs in the p type layer 3, which then migrate across the substrate 7 and into the n type layer 5, producing current. The electron hole carriers are quite mobile and will usually migrate large distances without recombination. The current produced by the incident photons is split into two output currents, I1 and I2. The level of current at each output is inversely proportional to the distance from the output terminal I1 or I2 to the spot of incident light. Current at terminal I1 is at a maximum when the spot is closest to I1, which occurs when the distance from I1 to the spot is zero. Similarly, the current output at terminal I2 is maximum when the distance from terminal I2 to the spot is zero. By comparing the two currents I1 and I2, it is then possible to generate a linear position value for the spot of light focused onto the photosensor. Typically the output is expressed as a distance which is proportional to the distance of the light spot from one end of the sensor. Position sensitive detectors using this approach are currently available from Hamamatsu, part no. S3271, and Toshiba, part no. TPS 710, as only two examples.
The prior art PSD of FIG. 1 uses a common terminal at the n-type layer, which is contacted at the backside of the device. It also requires a special high resistivity silicon substrate. These characteristics make it incompatible with the state of the art semiconductor processes used to produce CMOS or BiCMOS linear and digital circuitry. This incompatible process type means that the photosensor and the other circuitry associated with autofocus systems, such as signal processing functions, control logic, LED drivers, the analog comparator needed to generate the linear position for the spot, and the A/D converter associated with the compare function, must all be placed on another semiconductor device separate from the PSD itself. To produce small, low cost cameras with autofocusing features, it is desirable to place all of the semiconductor components on a single integrated circuit.
Another prior art approach is implemented by Texas Optoelectronics using a bipolar semiconductor process. This PSD was developed for use in surveying equipment and leveling and grade measurement tools. A pair of opposing photodiodes is placed into a rectangular sensor area. The sensor looks like a rectangle that split along a diagonal. Each photodiode is a triangular region of semiconductor material of p or n type, which is formed over a region of the opposing type of semiconductor material to form a P-N or N-P junction. Each photodiode has a single current output terminal at its widest end. The photodiodes are positioned close together along their sloping sides so that at the center of the sensor, incident light hits an equal area of each photodiode. At the ends of the sensor region, the area receiving incident light is large for one photodiode, while the area of the other photodiode which receives incident light is quite small. This occurs due to the triangular shapes of the photodiodes. When a laser spot strikes the pair of triangular areas, two currents are generated. Each of these currents is inversely proportional to the distance of the laser spot from the respective terminal.
However, this prior art bipolar PSD requires that the spot used be of a particular size for accuracy. If the spot has a diameter that is smaller than the width of the rectangle, it may appear to be all incident on one photodiode. The output currents will then erroneously indicate the spot is at the far left or far right of the sensor. In fact the light spot is in another position closer to the center of the sensor. The spot size must therefore be carefully controlled to prevent errors at the outputs. The device is intended for costly surveying and grade measurement equipment, and in that environment the spot size requirement is acceptable; however, it is generally an undesirable limitation. The bipolar device is also not compatible with the CMOS, LinCMOS, and BiCMOS technologies currently used for ASICs and integrated circuitry that provide the associated circuits for the autofocus systems.
The prior art PSD devices are not compatible with the ASIC semiconductor processes currently used to produce the digital signal processing circuitry, digital circuitry, and linear circuitry required for a complete autofocus system. Accordingly, there is a need for a position sensitive detector that provides a photosensor which can be integrated into these processes so that a complete autofocus system can be produced on a single integrated circuit. The PSD should also be tolerant of a range of spot sizes to reduce the system cost and improve accuracy.